Changed my mind. Everything will return both the unitID and the GUID. Yes, it's trivially easy for addons to get the GUID from the unitID if they want it, but since the lib already has the GUID (everything is tracked by GUID internally) why not save a function call?

API functions will accept either a unitID or a GUID.

Working on LibStub-ifying it at the moment. Will fix the bugs with people leaving the group after that, and then work on refactoring the messy/redundant parts, and then add the callbacks and API.